This invention relates to a method and apparatus for the transfer of agglomerations of solid particulate matter suspended in a liquid to a recording surface.
One particular application to which this invention may be applied is the transfer of agglomerations of high intensity colouring materials to a recording surface for the purpose of non-impact printing. It is to be understood, however, that the invention is not limited to delivering coloured materials for the purpose of non-impact printing but may be used to deposit other materials in a defined pattern on a substrate. Examples of other applications include the delivery of biological materials for use in immunoassay, delivery of particulate drugs and deposition of phosphors or flurophosphors for security coding.
This invention, however, will be discussed in relation to its application to printing but its scope is broader than this.
There are a number of different forms of equipment used for the non-impact printing systems which are generally referred to as ink jet printing. It is usual for ink to be fed through a nozzle, the exit diameter of which nozzle being a major factor in determining the droplet size and hence the size of the resulting dots on a recording surface. The droplets may be produced from the nozzle either continuously in which case the method is termed continuous printing or they may be produced individually as required in which case the method is termed drop on demand printing. In continuous printing an ink is delivered through the nozzle at high pressure and the nozzle is perturbed at a substantially constant frequency which results in a stream of droplets of constant size. By applying charge to the droplets and using an electric field external to the nozzle selected droplets may be deflected in their passage to the recording surface in response to a signal effecting the electric field whereby forming a pattern on the recording surface in response to the control signal. Drop on demand printing operates by producing local pressure pulses in the liquid in the vicinity of a small nozzle which results in a droplet of liquid being ejected from the nozzle.
In either type of jet printing the colouring material is a soluble dye combined with binders to render the printed image more permanent. The disadvantage of soluble dyes is that the printed image density is not high enough in many applications and that the dyes fade under exposure in the environment. A further disadvantage with soluble dye materials is that the quality of the printed image is dependent on the properties of the recording surface. Pigmented inks are known to produce higher density images than soluble dyes and are also more permanent. Pigments may also be used in jet printers but the production of a dense image requires a high concentration of pigment material in a liquid carrier. The high concentration of pigment material affects the droplet breakup in continuous printers and results in less uniform printing. Drop on demand printers do not have a high continuous pressure and the droplet generation is strongly dependent on local conditions in the nozzle, therefore the presence of pigments can block the nozzle or otherwise modify the local nozzle conditions or block the nozzle such that droplets are not correctly ejected.
A further process, known as electrostatic ink jet printing, is characterised by an electrostatic pull on a liquid and is disclosed in for instance U.S. Pat. No. 3,060,429. This involves the generation and acceleration of charged droplets, from a nozzle containing the liquid, to a platen electrode by a high voltage being maintained between the nozzle and the platen. This process is further optimised by including a valving electrode that is used to interrupt or control the jet flow as well as two pairs of electrodes used to manipulate the flight path of the droplets. Ink jet printing is achieved by locating a paper substrate just prior to the platen electrode and using a conductive solution of ink.
It is an object of this invention to provide agglomerations of particulate material in a liquid which are not produced by a nozzle and hence the size of the agglomerations are not affected by the size of the nozzle.
It is a further object of this invention to provide agglomerations of particles in a liquid with a very high concentration of particulate material so that a high concentration of the particulate material can be provided onto a recording surface.
It is a further object of this invention to produce agglomerations of particles in a liquid using a liquid which is not necessarily conductive.